With the size reduction of transistors, integration density of semiconductor integrated circuits has been increased, and processing speed of electrical signals still continues increasing. Electrical signal wiring in semiconductor integrated circuits, particularly, global wiring, which is relatively long range wiring, has problems of wiring delay, degradation in signal quality (skew, jitter) and the like, and also problems of signal degradation due to crosstalk between electrical wirings adjacent to each other and of being susceptible to the noise from the surroundings of the semiconductor integrated circuit. The delay of global wiring has been suppressed by insertion of a repeater circuit. However, because of increase in integration density and in information processing amount, the suppression effect of a repeater circuit has been decreased, and additionally a bottleneck such as of increase in power consumption induced by insertion of a number of repeater circuits has been created.
In recent years, as one of methods for solving these problems, technology of partly replacing electrical wiring in a semiconductor integrated circuit by optical wiring has been proposed. As a result of such proposals, non-patent document 1 discloses a structure in which an optical wiring layer fabricated on a separate substrate is bonded onto an electrical wiring layer in a semiconductor integrated circuit.
In the optical wiring layer, arranged are an optical waveguide which guides optical signals, an optical modulator for converting electrical signals from the semiconductor integrated circuit into optical signals, a photodetector for converting optical signals having propagated and reached it into electrical signals, and the like.
The optical modulator converts electrical signals propagated electrically from an electrical wiring layer in the semiconductor integrated circuit into optical signals by using light from a light source located outside the semiconductor integrated circuit. In the similar sense, the photodetector converts optical signals having propagated to the electrical wiring layer into electrical signals.
The optical waveguides used in the optical wiring layer are arranged in high density by employing a core material having a high refractive index, such as silicon or silicon nitride, to achieve a large difference in refractive index between it and a cladding material, and thus to enable the effective bending radius to be smaller.
The dimension of a core layer in an optical waveguide is generally as small as about one micron to sub-microns. Accordingly, an optical waveguide has been optically connected with an optical fiber from a horizontal direction by employing a structure with an enlarged spot size on the optical waveguide side or using a lensed fiber having a lens effect given by processing its head. However, if horizontally plugging and unplugging an optical fiber or the like into and from a board provided with an optical wiring LSI are performed, an area for the plugging and unplugging needs to be provided, which has been a factor impeding high-density mounting.
In this respect, non-patent document 2 discloses a structure where an optical fiber is optically connected from above an optical waveguide. In the structure, the optical fiber is inclined by 10 degrees at a diffraction grating fabricated on the optical waveguide side, so as to reduce the amount of light propagating toward the opposite direction to the desired one (FIGS. 2, 14 and 15 in non-patent document 2).